Synthesis of blood group A and B (type 2) tetrasaccharides. A strategy with fucosylation at the last stage.

The traditionally used strategy for the synthesis of blood group A and B tetrasaccharides includes 2'-O-fucosylation of lactosamine followed by insertion of an α1-3 linked N-acetylgalactosamine or a galactose moiety. Here, we report the synthesis of 3-aminopropyl glycosides of A (type 2) and B (type 2) tetrasaccharides via an alternative sequence, i.e. α-galactosaminylation (or α-galactosylation) followed by α-fucosylation. This strategy allows us to synthesize fucose-free trisaccharides GalNAcα1-3Galß1-4GlcNAc and Galα1-3Galß1-4GlcNAc, which are promising targets for immunotherapy utilising human natural antibodies against the trisaccharides. The key stage in this scheme was the selective chloroacetylation of the 2'-OH group of ßGal in the intermediate trisaccharides having the second (3-OH) unprotected group.The protocol is suitable for multigram syntheses and its further scaling up.